1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus which is capable of rapidly recording or reproducing a large amount of data on or from a magnetic tape and can be used as a data streamer, for example.
2. Description of the Related Art
Traditionally, a reel driving type recording and reproducing apparatus employs three kinds of tape speed control systems.
A first kind of tape speed control system is constructed to control a speed of a magnetic tape by reading a speed control signal recorded on one track of the magnetic tape while the tape is running. The magnetic recording and reproducing apparatus employing this system is, herein, referred to as a first type apparatus.
A second kind of tape speed control system is constructed to control a speed of a magnetic tape based on a tape winding diameter of a take-up reel or a supply reel. The tape winding diameter is measured while the tape is running. The magnetic recording and reproducing apparatus employing this system is, herein, referred to as a second type apparatus.
A third kind of tape speed control system is constructed to control a speed of a magnetic tape based on a running speed of the magnetic tape detected by a speed detector like a rotary encoder. The magnetic recording and reproducing apparatus employing this system is, herein, referred to as a third type apparatus.
For explaining concrete construction of the reproducing and recording apparatus outlined above, as an example, the third type apparatus will be described hereinafter.
A cassette designed according to the DCAS format has two reels on which a magnetic tape is wound inside of itself. The cassette molded by injecting a resin has some guide pins which are formed integrally.
When the cassette is loaded to the recording and reproducing apparatus, a light-emitting diode is opposed to the back surface of the magnetic tape, while a photodiode, a first tape guide, a magnetic head, a second tape guide and a speed-detecting roller are opposed to the right surface of the magnetic tape. When recording or reproducing information on or from the magnetic tape, the first tape guide, magnetic head, the second tape guide and speed-detecting roller come into contact with the magnetic tape, in which case the speed detecting roller is rolling. The light-emitting diode and the photodiode between the magnetic tape is located are used for detecting a start point and an end point of the magnetic tape.
The speed-detecting roller is linked to a rotary encoder so that the rotary encoder can detect the running speed of the magnetic tape.
The recording and reproducing apparatus includes a closed-loop control circuit having a difference operational circuit, a crystal oscillator, a reel motor driving circuit, the light-emitting diode, and the photodiode, both of which are mentioned above.
The rotary encoder supplies an output signal to the difference operational circuit at which the output signal is subtracted from a reference speed output signal of the crystal oscillator.
The difference operational circuit supplies an output signal representing a deviation between the running speed of the magnetic tape and the reference speed. The output signal is supplied to the reel motor driving circuit in which the signal is converted into a voltage matching to the deviation.
The reel motor driving circuit supplies the resulting voltage to two reel motors through a switch by which the magnetic tape is allowed to be switched to one of the running directions.
The foregoing closed-loop speed control system is constructed to control the running speed of the magnetic tape in both directions. The magnetic recording and reproducing apparatus employing the foregoing arrangement is referred to as a fourth type apparatus for the convenience of the description.
The foregoing four apparatuses, however, have the disadvantages mentioned below.
The first type apparatus requires on a recording format included in the magnetic tape a dedicated track for recording a speed control signal. Hence, this apparatus is disadvantageous in light of high density recording and requires a costly magnetic tape because it is necessary to record a speed control signal on the magnetic tape in advance. Further, the apparatus also has a disadvantage that the speed cannot be precisely controlled if a drop-out takes place in detecting the speed control signal.
The second type apparatus is weak to an irregular thickness of the magnetic tape, an irregular winding of the magnetic tape provided on the reel and a dimensional error of the reel. The apparatus thus has a disadvantage that it is impossible to detect a running speed of the magnetic tape with high precision and stably control the speed of the magnetic tape.
The third type apparatus requires additional provision of a speed detector like the rotary encoder for detecting a running speed of the magnetic tape. The apparatus disadvantageously has large restrictions about reduction of an overall apparatus and lowering the cost.
The fourth type apparatus has a disadvantages mentioned below in addition to the aforementioned disadvantages.
That is, the guide pins which are integrally formed with the cassette molded by injecting a resin, are inevitably formed to be tapered since the metallic mold normally has a draft. The tapered form of the guide pins disadvantageously brings about vertical snaking of the magnetic tape. The first and second tape guides disallow the snaking to be positively suppressed since the guide pins are positioned inside of the first and second tape guides. Another method may be provided wherein the guide pins are made of ceramic and are mounted in the cassette as separate members. This method, however, is not substantially effective in overcoming the disadvantage if the change of the cassette allows the guide pins to be substantially inclined because the cassette is made of a resin material and a dimensional variation is caused among a plurality of cassettes. In other words, the mechanical construction of the fourth type apparatus has a large disadvantage that it cannot achieve stable running of the magnetic tape and secure the running precision of the magnetic tape without depending on the cassette.
The foregoing disadvantages are a practically large obstacle in keeping the capacity and the speed of the magnetic recording and reproducing apparatus larger and faster.